This invention relates generally to optical waveguides on ceramic substrates, and more particularly to forming a silica glass film on A12O3 ceramic substrates to be used as an optical waveguide.
As part of the background information for this invention, applicants incorporate by reference herein their previous U.S. patent application filed on Jun. 29, 1990 entitled "Apparatus and Method of Forming Optical Waveguides on Metalized Substrates" which has been granted U.S. Pat. No. 5,059,475. Flame hydrolysis deposition ("FHD"), a process which involves the deposit of powdered glass on a substrate and its subsequent fusion for consolidation into a solid glass at high temperatures, has been used to deposit waveguides for optical signal transmission on silicon and silica substrates. Other deposition methods such as chemical vapor deposition ("CVD") and sputtering have been widely used to form optical waveguides on substrates of silicon and silica. Previous technologies involve silica waveguides made on silicon or silica substrates. But this is limiting to the integration of optics and electronics since electronic integrated circuits are usually bonded onto ceramic substrates. Also, since the melting temperature of silicon is 1410.degree. C. and the softening temperature of silica is around 1300.degree.-1500.degree. C., the waveguide fabrication temperature is limited below these temperatures. A ceramic substrate permits much higher process temperatures, up to 1600.degree.-1700.degree. C., for the fabrication of waveguides thereon.
The present invention offers several advantages over previous technology. Silica integrated optical circuits can now be applied to A12O3 substrates. Since A12O3 is a popular electronic substrate, forming optical waveguides on this type of substrate enables integration between optics and electronics. Furthermore, ceramics can withstand higher process temperatures, and normally, better quality silica glass is made at higher temperature. The present invention enables glass with a high thermal expansion coefficient to be deposited onto a substrate made of ceramics. The resulting glass film stress is low to allow long-term stability against film peeling or cracking which often happens on conventional high silica FHD glass.
Another advantage of the invention is that it can be applied to glaze ceramic plates or parts. The present invention is useful as an optical interconnection that can solve bottlenecks of electrical connections in high speed electronics. The present invention involves the formation of optical waveguides on A12O3 substrates. The A12O3 substrates may also have electronic or optoelectronic IC chips, to perform as optical interconnects. Therefore, the present invention is useful in making optoelectronic devices for communication and computer applications.
The present invention is a unique formation of glass on A12O3 substrates. The glass film can be used as an optical waveguide. The glass film may be deposited directly onto the ceramic substrate or onto a coating on the substrate. The glass composition in the present invention is 35-90 mol % SiO2. With this particular glass composition, the thermal expansion mismatch between the glass film and substrate will be low enough to avoid peeling or cracking of the glass film. The FHD process may be used to form the silica glass film on the substrates. The thermal expansion coefficient of the glass film is generally between 15.times.10.sup.-7 dyne/cm.sup.2 at 90 mol % SiO2 and 70.times.10.sup.-7 dyne/cm.sup.2 at 35 mol % Sio2. Refractive index raising dopants such as GeO2 and TiO2 are used to make layers with different refractive indices. The glass film can also be used to glaze ceramic parts. Both thin film and thick film application A12O3 substrates can be deposited with these glass films to form waveguides and integrated optical circuits for optoelectronic integration.
The foregoing and other objects and advantages of the invention will become more apparent when considered in view of the accompanying drawings and the following description: